Conventionally, as shown in FIG. 15, a collecting lens 101 is known which has a flat surface 110 as a first surface and a hyperboloid surface 120 as a second surface, and in which the rotational axis C of the hyperboloid surface 120 is inclined so as to form an angle θ with the normal line H of the flat surface 110 (Japanese Examined Patent Application Publication No. H7-36041; called Patent Document 1). In a collecting lens 101 of the configuration shown in FIG. 15, light rays incident at a certain angle δ to the rotational axis C become parallel to the rotational axis C of the hyperboloid surface 120 within the collecting lens 101, and are focused aplanatically on a focal point F. The angle δ is an angle satisfying Snell's law, or in other words, sin (θ+δ)=n sin θ given n as the refractive index of the collecting lens 101. Thus, with the collecting lens 101 shown in FIG. 15, it is possible to suppress the occurrence of off-axis aberrations and collect light rays efficiently from directions oblique to the normal line H of the flat surface 110.
The collecting lens 101 disclosed in Patent Document 1 is designed for infrared light rays, and Patent Document 1 discloses using polyethylene as the lens material.
Patent Document 1 also describes a multi-segment lens with an array of a plurality of collecting lenses all having a focal point in the same position.
As shown in FIGS. 16A and 16B, moreover, Patent Document 1 also proposes making collecting lens 101 a Fresnel lens, and making the common rotational axis C of hyperboloid surfaces 121, 122 and 123 of the second surface intersect the flat surface 110 (the first surface) obliquely in order to suppress the occurrence of off-axis aberrations. In this case, hyperboloid surfaces 121, 122 and 123 constitute the lens surface.
In the collecting lens 101 of FIGS. 16A and 16B, Patent Document 1 describes the possibility of providing an angle between the normal line H of the flat surface 110 and parallel light rays that are focused aplanatically on a focal point, depending on the angle formed by the flat surface 110 and the common rotational axis C of hyperboloid surfaces 121, 122 and 123. It is thus possible to suppress the occurrence of off-axis aberrations and collect light rays efficiently from directions oblique to the normal line H of the flat plane 110 in the collecting lens 101 of FIGS. 16A and 16B.
Moreover, an axicon lens has been proposed whereby the focal depth can be increased over that of an axicon lens with a single conical surface (Japanese Unexamined Patent Application Publication No. 2009-082958; called Patent Document 2).
As shown in FIGS. 17A, 17B and 17C, Patent Document 2 describes preparing three lens members 221a, 221b and 221c with different conical surfaces as lens surfaces having different angles relative to a optical axis OX, and arranging the three lens members 221a, 221b and 221c in the light axial direction as shown in FIG. 17D to thereby obtain a greater focal depth than with an axicon lens having a single conical surface. Although the three lens members 221a, 221b and 221c are assembled to obtain an axicon lens having three conical surfaces in Patent Document 2, an axicon lens has also been proposed in which lens surface 213 is constituted with a spline curve (free curve) as shown in FIG. 17E. The focal depth can also be increased over that of an axicon lens having a single conical surface in an axicon lens constituted as shown in FIG. 17E.
Moreover, an optical sensing device has been proposed, provided with a multi-segment lens including a plurality of lenses with roughly the same focal position assembled on a single flat surface, together with an infrared sensing element that is a light-receiving element disposed at the aforementioned focal position (see for example Japanese Patent Publication No. 3090336 (called Patent Document 3) and Japanese Patent Publication No. 3090337 (called Patent Document 4)).
In the multi-segment lenses disclosed in Patent Documents 3 and 4, the first surface is a flat surface and the second surface is a hyperboloid surface having a main axis that intersects the normal line of the first surface obliquely. Moreover, the multi-segment lenses disclosed in Patent Documents 3 and 4 are designed for use with infrared light rays, and polyethylene is disclosed as the lens material. Moreover, Patent Documents 3 and 4 describe preparing the multi-segment lenses by injection molding.
In the collecting lens 101 of FIG. 15 disclosed in Patent Document 1, the rotational axis C of the hyperboloid surface 120 intersects the normal line H of the flat surface 110 obliquely, so hyperboloid surface 120 is not rotationally symmetrical relative to the normal line H of the flat surface 110. This makes the collecting lens 101 and the mold for the collecting lens 101 difficult to prepare by rotary forming using a lathe or the like.
In the collecting lens 101 of FIGS. 16A and 16B, moreover, the rotational axis C of the hyperboloid surfaces 121, 122 and 123, which constitute the output surface, is at an oblique angle to the normal line H of the flat surface 110, which is the input surface, so the hyperboloid surfaces 121, 122 and 123 are not rotationally symmetrical relative to the normal line H of the flat surface 110. This also makes the collecting lens 101 and the mold for the collecting lens 101 difficult to prepare by rotary forming using a lathe or the like.
Therefore, when preparing the collecting lenses 101 and molds for the collecting lenses 101 of FIG. 15 and FIGS. 16A and 16B above, the hyperboloid surfaces 120, 121, 122 and 123 or curved surfaces corresponding to these hyperboloid surfaces 120, 121, 122 and 123 must be formed using a fabricating machine with multiaxial control, by bringing only the cutting edge of a sharp cutting tool 130 with a nose radius (also called a corner radius) of a few microns into point contact with a work piece 140 and cutting it with a very fine pitch as shown in FIG. 18. The work piece 140 is a base material for forming the collecting lens 101 directly, or a base material for forming the mold. Consequently, the working time for preparing the collecting lens 101 or the mold for collecting lens 101 is prolonged, increasing the cost of the collecting lens 101.
Moreover, in the aforementioned multi-segment lenses the second surface of each lens is a hyperboloid surface intersecting the normal line of the first surface obliquely, and is not rotationally symmetrical relative to the normal line of the first surface, making the molds for the multi-segment lens difficult to prepare by rotary forming using a lathe or the like.
In addition, the axicon lens disclosed in Patent Document 2 cannot focus light rays incident from a direction oblique to the direction of lens thickness on a focal point. That is to say, the axicon lens disclosed in Patent Document 2 has a different purpose from one such as the collecting lens 101 disclosed in Patent Document 1, which focuses light rays incident at an angle to the first surface (flat surface 110) on a focal point F, or in other words from the collecting lens 101, in which the optical axis intersects the direction of lens thickness obliquely (at an angle).